1. Field of the Invention
The present invention is related to electrical medical apparatus, and more particularly, to a battery impedance measurement device for use in an implanted battery powered medical apparatus with telemetering capabilities.
2. Description of the Prior Art
It is well own in the field of implantable medical apparatus that one of the most critical decisions to be made by the medical caretaker is when to replace the implanted battery. Many systems for determining an end-of-life (EOL) signal have been developed and disclosed which alleviate one aspect of the problem; that is, the battery must be replaced as soon as reasonably possible after the EOL signal. However, with the advent of improved batteries which provide many years of life to the implanted medical apparatus, the need has arisen to have accurate information on just how much longer the battery will last under its normal loading, as opposed to a simple EOL signal.
Various prior art circuits have been developed to provide the desired information, such as those disclosed, by way of example, in U.S. Pat. Nos. 4,231,027; 4,259,639; and 4,324,251. While part of the teachings of this prior art is the well recognized fact that the remaining battery life is directly related to the existing internal battery impedance, none of these teachings disclose a circuit for measuring the battery impedance with the accuracy and manufacturing conveniences of the present invention.